In electrical apparatus in which safety requirements are very stringent, for example in safety barriers according to German Engineering Standard VDE 0170/0171, the required fuse has until now been inserted and encapsulated in such a way that the subsequent changing of a burned fuse is not possible, rendering the entire mechanism useless. This is done to prevent the improper repair and exchange of fuses such as the substitution of frequently burnt-out fuses by those of a higher rating.
Known glass tube heat coil fuses can be substituted in this manner without difficulty, since the dimensions of the casing are the same, regardless of rating and tripping mechanism.
To prevent the substitution of fuses with a lower rating by those with a higher rating, one measure--as far as domestic fuses are concerned--is to vary the diameter of the end face according to the rating of the fuse. In such cases the socket is designed with a ceramic ring around one of its contacts, so that only fuses with a sufficiently small end face can touch the electrical contact.
Because of necessary manufacturers' tolerances on the one hand and a certain minimum stability on the other hand, even with miniaturized fuses, only relatively few current ratings can be encoded in that fashion.
Because of the cylindrical design not only of domestic fuses but also of glass tube fuses, it is very easy to carry out unauthorized repairs of inserting solid pieces of wire into the sockets.
According to German Utility Model 84 11868 a miniature fuse is known in whose fusible insert some contact pins are connected mechanically and with electric conductivity, pointing radially outward through openings in the area where the casing sleeves are joined.
Since this miniature fuse is meant to be soldered into printed circuit boards, the contact pins are adapted to fit the holes in the boards, and assembly errors are prevented by varying the spacing of the contact pins. This means that only those miniature fuses can be inserted in the printed circuit board in which the spacing of the contact pins matches the spacing of solder on the printed circiut board.
However, on account of miniaturization, this method does not lead to a very great choice in variety, because the solder points on printed circuit boards normally have a grid spacing of 2.5 mm. Thus the spacing variation of the contact pins in miniature fuses has to follow the 2.5 mm grid.